In contrast to terrestrial photosynthesis, photosynthesis in aquatic plants is not limited by water loss through open stomata but rather by C02 uptake. In most cases, C02 diffuses directly through the outer cell walls (epidermis) of aquatic plant leaves. Even though the C02 concentration in water is roughly the same as in air, dissolved gases diffuse 10^4 time more slowly in water. Also, the boundary layer of water surrounding aquatic leaves is about ten times thicker than the air layer that surrounds terrestrial plants. To cope with a thick boundary layer of water around their leaves and the slower diffusion of C02 in water, aquatic plants have developed specialized mechanisms. For instance, some aquatic plants have a CO2-concentrating mechanism that, while different from C4 terrestrial plants, serves a similar purpose. Aquatic plants also engage in the active transport of HC03 (bicarbonate - a form that carbon takes when dissolved in water) as an additional carbon source. Also, C4-like pathways have been identified in waterthyme (hydrilla verticillata) and a marine diatom. CAM pathways have been discovered in aquatic plants as well, although it less clear what advantage this confers in an environment where water shortage is not a problem. Most aquatic plants, like terrestrial ones, engage in C3 photosynthesis.
Aquatic plants have other adaptations to deal with thick boundary layers and slow C02 diffusion present in aquatic environments. Some aquatic plants uptake CO2 from the sediments in which they are anchored. Quillworts, for example, which live in carbon-poor lakes, receive 60-100% of their carbon from lake sediment via their roots. Aquatic plants also often have very thin leaves, resulting in a higher surface area to volume ratio and decreasing the thickness of the unstirred boundary layer. It is also interesting to note that, because light energy diminishes rapidly with water depth, deeper leaves on some aquatic species exhibit characteristics similar to those of shade plants. For instance, aquatic plant chloroplasts are often located in the top cell layer of leaves to ensure that as much light as possible is absorbed, and the optimum net production (photosynthesis minus respiration) of aquatic plants is tweaked for low-light conditions. Finally, because absorption of light in water is much more wavelength-specific than in air, it is thought that aquatic plants are more flexible in their wavelength requirements than terrestrial ones.
The above summarized from Plant Physiological Ecology by Lambers and Chapin, pp. 82 - 85, and
"Water Plants 101" http://www.thekrib.com/Plants//phisio.html